LETTERIEICE Electronics Express, Vol.13, No.3, 1–7

Observation of Brillouin gain
spectrum in optical ﬁbers intelecommunication band:
Effect of pump wavelength
Heeyoung Leea), Neisei Hayashi, Yosuke Mizuno,
and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
Yokohama 226–8503, Japana) hylee@sonic.pi.titech.ac.jp

Abstract: We study the optical wavelength dependence of the Brillouingain spectrum for a silica single-mode ﬁber in the range from 1511 to1555 nm using a measurement setup containing an erbium-doped ﬁber
ampliﬁer (EDFA) in the Brillouin pump path. The Brillouin peak power
and linewidth are clearly dependent on the pump wavelength, the trends ofwhich correspond well with the wavelength dependence of the EDFA gain.
This result suggests that we need to consider the unique wavelength dependence of the EDFA gain to practically obtain a higher Brillouin peak
power and narrower linewidth at the telecommunication wavelength.
Keywords: Brillouin scattering, optical ﬁbers, erbium-doped ﬁber ampliﬁers
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
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1© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 2016

Introduction

Over the past few decades, optical ﬁber sensors based on Brillouin scattering havebeen extensively studied owing to their ability to measure strain and temperature
distribution [1, 2, 3, 4, 5]. Brillouin scattering is known to be one of the mostsigniﬁcant nonlinear phenomena in optical ﬁbers [6]. Higher-power pump light
induces even higher-power scattered light, leading to a high signal-to-noise ratio(SNR) of the sensing system [7, 8, 9, 10]. In addition, such higher-power pumplight is reported to provide narrower Brillouin linewidth, resulting in a highprecision of the measurement [11]. Therefore, Brillouin sensors have been mainly
implemented using light sources at the telecommunication wavelength, whererelatively inexpensive erbium-doped ﬁber ampliﬁers (EDFAs) have been used,though some exceptions have been reported [12, 13].
The typical wavelengths of laser diodes at the telecommunication wavelength
are 1510, 1530, and 1550 nm [14, 15, 16, 17]. To date, no reports have been
provided as to which wavelength is the most suitable for Brillouin observation,
because the Brillouin gain spectrum (BGS) is known not to be largely dependent on
the wavelength of the pump light in this range [6]. However, the BGS observed byusing a practical setup containing an EDFA for preparing the Brillouin pump lightshould exhibit optical wavelength dependence if we consider the wavelength
dependence of the gain of the EDFA [18, 19, 20].
In this work, we investigate the optical wavelength dependence of the BGS in asilica single-mode ﬁber (SMF) using an experimental setup containing an EDFA in
the Brillouin pump path. The driving current of the 980-nm pump laser in the2

IEICE Electronics Express, Vol.13, No.3, 1–7

EDFA is constant. A tunable laser at 1530–1555 nm and a wavelength-ﬁxed laser
at 1511 nm are used. The Brillouin peak power and linewidth are found to exhibit
large dependences on the pump wavelength, the trends of which are in good
agreement with the wavelength dependence of the EDFA gain.
2

Principle

The pump light injected into an optical ﬁber interacts with acoustic phonons andgenerates backscattered light with a frequency downshift. This phenomenon istermed Brillouin scattering, and the scattered light spectrum is the BGS. The
amount of the frequency downshift, referred to as the Brillouin frequency shift(BFS) B , is given by [6]
B ¼

2nva
;
p

ð1Þ

where n is the refractive index, va is the acoustic velocity, and p is the pumpwavelength. The BFS value is known to be ∼11 GHz in silica SMFs at a pumpwavelength of 1550 nm. In principle, the peak power of the BGS is inﬂuenced by
the Brillouin gain coefﬁcient gB , which is given by [6]gB ¼

2n7 p12 2;cp 2 va B

ð2Þ

where p12 is the longitudinal elasto-optic coefﬁcient, c is the velocity of light invacuum, ρ is the density of the ﬁber core, and B is the Brillouin linewidth. In
silica SMFs, gB is reported to be 3{5  1011 m/W. When p varies from 1511 nm
to 1555 nm, the change in gB is negligible (as shown below).
The simplest method for obtaining higher-power Brillouin-scattered light is byincreasing the power of the pump light, which can be easily done at the telecommunication wavelength by amplifying the pump light using an EDFA. Fluorescence in an EDFA occurs through the radiative transition of erbium ions between
different energy levels—speciﬁcally, from 4 I13=2 to 4 I15=2 in a 980-nm pumping
conﬁguration—and exhibits a clear wavelength dependence (note that different
EDFAs exhibit different wavelength dependences of the gain) [18]. Consequently,
when an EDFA is used in the pump path, the Brillouin peak power is signiﬁcantly
dependent on the wavelength.
3© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 2016

Experimental setup

We employed a 1-km-long silica SMF (Brillouin threshold = 17 dBm [6]) as a ﬁber
under test (FUT). The experimental setup shown in Fig. 1, which is basically thesame as that previously reported [21], enabled us to observe the BGS with a highfrequency resolution using self-heterodyne detection. As light sources, we employed a tunable laser at 1530–1555 nm and a wavelength-ﬁxed laser at 1511 nm,
the output powers of which were set to 6 dBm (¼ 4 mW). The linewidth of the twolasers were lower than 1 MHz. An EDFA, which operated in an automatic-currentcontrol mode, was inserted in the Brillouin pump path; the reference light and theBrillouin-scattered light were not ampliﬁed. The electrical signal was also notampliﬁed before it was monitored using an electrical spectrum analyzer (ESA). The

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IEICE Electronics Express, Vol.13, No.3, 1–7

end of the FUT was cut at an angle to suppress the Fresnel reﬂection. The
polarization state was averaged using a polarization scrambler inserted in thereference path. The room temperature was maintained at 22 °C.

Fig. 1.

4

Experimental setup for observing the BGS. EDFA, erbiumdoped ﬁber ampliﬁer; ESA, electrical spectrum analyzer; FUT,
ﬁber under test; PC, personal computer; PD, photodiode;
PSCR, polarization scrambler

Experimental results

We veriﬁed that the laser output power and the photodiode (PD) sensitivity do notlargely depend on the optical wavelength. Fig. 2(a) shows the output spectra of thetwo lasers. The peak power was almost the same irrespective of the wavelength.
The spectral ﬂoor of the 1511-nm laser was not ﬂat, but had no quantiﬁable
inﬂuence on the Brillouin measurement because the maximum power of the ﬂoor
was more than 40 dB lower than the peak power. Fig. 2(b) shows the measured
BGSs corresponding to the laser outputs at each wavelength in Fig. 2(a).
The EDFA in the pump path was removed in this measurement so that thisresult can be interpreted as the wavelength dependence of the PD sensitivity. The
PD sensitivity had a slight wavelength dependence (0.3-dB change in the power(a)© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 2016

Fig. 2.

(b)(a) Output spectra of a tunable laser at 1530–1555 nm and awavelength-ﬁxed laser at 1511 nm. (b) BGSs measured without
an EDFA in the pump path.

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IEICE Electronics Express, Vol.13, No.3, 1–7

Fig. 3.

Output spectra from the EDFA measured at laser wavelengths
from 1511 to 1555 nm.

(a)Fig. 4.

© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 2016(b)(a) BGSs measured at laser wavelengths from 1530 to 1555 nm.
The inset shows a magniﬁed view of the dip at ∼11.0 GHz.
(b) BGS measured at 1511 nm; no signal was detected.

in the wavelength range), which was negligible compared to the wavelength
dependence of the EDFA gain discussed below.
We then measured the output spectra from the EDFA with varying wavelengthsof the lasers (Fig. 3). The driving current of the 980-nm pump laser in the EDFA
was set to 700 mA, corresponding to an output of ∼13 dBm at 1550 nm; note that
this value is lower than the Brillouin threshold of ∼17 dBm. Each laser output
spectrum was ampliﬁed on the ampliﬁed spontaneous emission (ASE) spectrum[18]. The peak power of the laser output was dependent on the optical wavelength;with increasing wavelength from 1511 to 1555 nm, the peak power increased, but a
dip was observed at 1535 nm.
Subsequently, we observed the BGSs when the laser wavelength was changed
from 1530 to 1555 nm (Fig. 4(a)). The BGS, which should appear at ∼11.15 GHz ata wavelength of 1511 nm according to Eq. (1), was not observed because it was
hidden by the noise ﬂoor (Fig. 4(b)). As the wavelength increased, the BFS shiftedto a lower frequency, which is natural according to Eq. (1). With increasing
wavelength, the Brillouin peak power increased; such a clear wavelength dependence (>15-dB change in power in the wavelength range) was not observed
for the PD sensitivity (Fig. 2(b)). A small but clear dip was observed at 1535 nm,

5IEICE Electronics Express, Vol.13, No.3, 1–7

Fig. 5.

Brillouin linewidth calculated at laser wavelengths from 1530
to 1555 nm. Note that the laser wavelength is inversely
proportional to the BFS.

which agrees well with the wavelength of the EDFA output at which the dip
occurred. The Brillouin peak power did not exhibit a completely linear dependence
on the peak power of the EDFA output, because the Brillouin peak power
exponentially depends on the pump power even in the range lower than the
Brillouin threshold [6].
Finally, the Brillouin linewidth dependence on the laser wavelength in the range
from 1530 to 1555 nm was calculated using Fig. 4(a). After converting the verticalaxis into a linear scale, each BGS was ﬁtted to a Lorentzian function, the linewidthof which was deﬁned as the Brillouin linewidth in this calculation. This deﬁnition isvalid because the linewidth of the tunable laser was lower than 1 MHz [11]. The
result shown in Fig. 5 indicates that the Brillouin linewidth becomes narrower as
the laser wavelength increases; this trend agrees well with the Brillouin linewidth
narrowing effect detailed in Ref. [11]. The instability of the calculated values at∼1535 nm (corresponding to the BFS of ∼11.0 GHz) may originate from thedecrease in the Brillouin peak power, but the BGSs at ∼1535 nm were relatively
so close to the noise ﬂoor that it was difﬁcult to calculate the Brillouin linewidths
with a sufﬁciently high accuracy.
5

Conclusion

The optical wavelength dependence of the BGS in a silica SMF was investigated in
the wavelength range from 1511 to 1555 nm using an experimental setup containing an EDFA in the Brillouin pump path. The Brillouin peak power and linewidth
exhibited a clear dependence on the pump wavelength, the trend of whichcorresponded well with the wavelength dependence of the EDFA gain. Thus, we
showed that, to practically obtain a higher Brillouin peak power and narrower
linewidth at the telecommunication wavelength, the unique wavelength dependenceof the gain of the EDFA used in the system should be carefully considered. We
anticipate that this result will be a practically useful guideline for selecting suitable
light sources for developing strain and temperature sensors based on Brillouinscattering with a high SNR and high precision.
© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 2016

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IEICE Electronics Express, Vol.13, No.3, 1–7

Acknowledgments
This work was supported by JSPS KAKENHI Grant Numbers 25709032,
26630180, and 25007652, and by research grants from the Iwatani Naoji Foundation, the SCAT Foundation, and the Konica Minolta Science and Technology
Foundation.

© IEICE 2016
DOI: 10.1587/elex.13.20151066
Received December 15, 2015
Accepted December 25, 2015
Publicized January 15, 2016
Copyedited February 10, 20167